Study Outline: Beyond Mendel's Genetics

Extending Mendel's
Genetics

Dominance does not always apply the way Mendel
charcterized it, and alleles may be incompletely dominant or
codominant. Each chromosome behaves genetically as if it were
composed of genes linked together in a linear order. Genes that are
in the same chromosome are said to be linked and may not undergo
independent assortment. Recombination of linked genes can occur as a
result of crossing over (breaking and joining of homologous
chromosomes during prophase I of meiosis).

1. Some heterozygous genotypes result in
incomplete dominance. Such individuals have an appearance that is
intermediate between the phenotypes of the two parents.

2. There are other variations in
dominance/recessiveness relationships. In codominance, a heterozygous
organism expresses both phenotypes of its two alleles. The type of
dominance we observe often depends on the size scale used for
examination.

3. Many genes have multiple (more than two)
alleles, like the gene for the human ABO blood groups which occupies
a single locus on the chromosome. (see handout)

4. Pleiotropy is the ability of a single gene to
affect multiple phenotypic traits. Most genes have many different
effects on an organism

5. In epistasis, one gene interferes with, or some
way regulates the expression of another gene.

6. Certain characters, such as human skin color
or height, are quantitative characters that vary in a continuous
fashion, indicating polygenic inheritance, an additive effect of two
or more genes on a single phenotypic character. [Multiple
independent pairs of genes may have similar and additive effects on a
particular phenotype.] In polygenic inheritance, the F1
generation is intermediate between the two parental types and shows
little variation. The F2 generation shows wide variation between the
two parental types.

7. Environment also influences quantitative
characters. Such characters are said to be multifactorial.

Mendelian Inheritance in Humans

The sex of humans and many other animals is
determined by the X and Y sex chromosomes or their equivalent.
Chromosomes that are not sex chromosomes are called autosomes.
Normal female mammals have two X chromosomes; normal males have one X
and one Y. The Y chromosome in mammals appears to be responsible for
determining male sex. (See article on SRY gene.) The X chromosome
contains many important genes that are unrelated to sex determination
and are required by both males and females. A male receives all of
his X-linked genes from his mother. A female receives X-linked genes
from both parents.

1. Family pedigrees can be used to deduce the
possible genotypes of individuals and make predictions about future
offspring. Any predictions are usually statistical probabilities
rather than absolute statements.

3. Consanguineous matings (inbreeding) between
close relatives may increase the chance that the offspring will be
homozygous for a rare deleterious allele.

4. Although they are far less common than the
recessive type, some human disorders are due to dominant alleles.
Dominant alleles that are lethal may kill the organism as an embryo
or act later, as in Huntington's disease. Outbreeding, mating of
totally unrelated individuals, increases the probability that the
offspring will be heterozygous at many loci. These heterozygous
individuals may be stronger and better able to survive than either
parent (hybrid vigor).

5. Using family histories, genetic counselors aid
couples in determining the odds that their children will have genetic
disorders. For certain diseases, tests that can identify carriers can
more accurately define those odds.

6. Once a child is conceived, the techniques of
amniocentesis and chorionic villi sampling can help determine whether
a suspected genetic disorder is present.

7. Medical researchers are just beginning to sort
out the genetic and environmental components of multifactorial
disorders, such as heart disease and cancer.